The following abbreviations are defined as follows:
2GSecond generation mobile system, such as GSM3GThird generation mobile system, such as UTRANCCCHCommon Control ChannelCNCore NetworkCS domainCircuit Switched domain, typically normal call trafficGSMGroupe Special Mobile, global mobile communication systemLALocation AreaLAILocation Area Identity, PLMN identifier + LA identifierLRLocation Registration, CS location update/PS routing area update, or combinedPBCCHPacket Broadcast Common ChannelPCCCHPacket Common Control ChannelPLMNPublic Land Mobile NetworkPSI3-seriesPacket System Information Type 3PS domainPacket Switched domain, connectionless trafficRANRadio Access NetworkRATRadio Access TechnologySISystem InformationUEUser Equipment, (e.g., mobile terminal or mobile station)UTRANUniversal Terrestrial Radio Access NetworkWCDMAWideband Code Division Multiple Access
A problem can arise when a 3G network operator, who does not control a 2G network, has established a national roaming agreement to provide service for network subscribers outside the coverage of the network operator's 3G coverage. The roaming partner may also provide 3G service. The 3G service, though, may not be available for national roaming. Similarly a dual mode network operator may allow international roaming only on the 2G network. Another similar case can occur when only specific home network customers have access to the 3G network. Roaming subscribers (generally those who do not have permission to use 3G service) should not use the 3G service when there are 3G cells within a location area. On the other hand, as 3G neighbor cells are broadcast on the 2G network, the roaming subscriber will also receive this information and use it as defined. This means that roaming subscribers may possibly reselect to the 3G cells. The network operator may assure that the roaming subscriber does not obtain 3G service by concentrating the 3G cells in location area(s) which are different from the 2G location area(s), and then by rejecting a location update request into a 3G location area for those subscribers who are not permitted to use 3G cells (with an appropriate cause value). The subscriber is then forced to leave the 3G cell, but stay in the same network when the reject cause is, for example, one specified as #15 (“no suitable cells in this location area”).
A similar (non-roaming) scenario may also occur in the user's 2G- and 3G-capable home network when the user, by subscription, is allowed to operate only in the 2G portion of the home network and not in 3G portion of the home network
In general, this means that if an attempted LR procedure ends in rejection by the network, then the behavior of the UE is governed by the cause value that the network included in the message indicating rejection of the request. In some cases the reject cause values are meaningful across all cells in the same Location Area (LA). As an example, cause value #15 (“no suitable cells in this location area”) denies access for the UE to all cells in the same LA. The location area reject forces the UE to add the location area to the list of forbidden location areas that are not accessible by the UE for a certain amount of time (such as 12 hours).
A future cell re-selection procedure will then exclude all cells of the serving Public Land Mobile Network (PLMN) that indicate the same LA, so in practice the UE is commanded by the network to search a cell of another LA in the same PLMN.
One problem with the current approach is that when the UE has re-selected back to a GSM cell, the only knowledge it has concerning WCDMA neighbor cells, based on the system information messages, is the radio frequency and scrambling code. In addition, the 2G to 3G reselection follows a principle that the suitability criteria of the target cell need not be checked before the reselection attempt is initiated. In some cases this is not possible without compromising the operation at the serving cell.
This means that the UE may, after a short time, try reselection to the same or another 3G cell. The outcome from this reselection would be a failure, as the location area is listed on the list of forbidden location areas. This though is not known unless the UE reselects to the cell or reads the 3G cell location area information (e.g. prior to the cell change). This approach is time consuming, increases power consumption and, in a worst case, the user may experience an inferior connection, as a service interrupt is generated when an unsuccessful (unnecessary) reselection attempt is performed during GPRS packet transfer mode (when NC0 or NC1 mode is applied for cell reselection). In this case, data transfer may be significantly degraded (depending on the frequency of “unnecessary” reselection attempts), a page can be lost, or the entire connection can be lost.
On the 2G side, the UE when camped on a CCCH is expected to read SI3/4 before reselection is possible. The information received in this SI (which may also be referred to as System Information) message contains information about the neighbor cell's location area. Thereby the UE can evaluate, before the cell change, whether the cell is listed on a list of forbidden location areas. If this is the case, the UE can prevent a cell change towards that cell.
In the situation of 2G-3G inter-working the same approach is not possible in most cases. The only currently specified solution for the UE is to attempt to perform cell re-selection as described above. This trial and error mechanism is not very efficient. The failure may continuously repeat even if the UE attempts multiple cell re-selections towards the same 3G cell.
In case of reselection towards a 3G cell, the UE needs to read the system information of the 3G cell in order to obtain the location area information of that cell. However in order to do this while there is an ongoing GPRS data transfer connection, the MS needs to interrupt the data transfer, which may cause the data throughput to be reduced.
It can be noted that a similar situation exists also on the 2G side when the UE or mobile station (MS) is camped on the PCCCH. In this case the UE receives the neighbor cell(s) parameters regarding cell reselection from the current serving cell's broadcast PSI3-series on the PBCCH. This means that the MS does not need to read the neighbor cell's SI3/4 information in order to obtain the parameters affecting cell reselection, so the MS will not know the LAI of the neighbor cell, thereby introducing the same situation as in 2G-3G inter-working cell reselection.